Optical slit



P 1934- M. BOWMAN-MANIFOLD ,9

OPTICAL SLIT Filed June 11, 1929 JOIT :muentor Gttomeys Patented Sept. 4, 1934 OPTICAL SLI'I Michael Bowman-Manifold, Worplesdon, England, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 11, 1929, Serial No. 370,113 In Great Britain July 16, 1928 5 Claims.

The present invention relates to optical slits such as may be used in recording and reproducing sounds.

The invention is applicable whether the record 5 in question is actually a record of" sounds or whether it is a record of any other relatively low frequency oscillations.

When a record of the modulations of intensity of a beam of light is made upon a travelling photographic film, the record being in the form of variations in density of the film, it is well known that the light should be caused to fall upon the sensitive surface in the form of a thin line transverse to the direction of movement of the film. The same applies where the record produced is in the form of variations in breadth of -a band 'of constant density. In order to obtain a uniform photographic effect (disregarding the sensitivity of the film) at all frequencies, it would and even to obtain uniformity over the important acoustic range of irequencies, a very thin line is required, with consequent reduction in the total light falling upon the sensitive surface.

It will be found, for example in reproducing, that when the ripple wave length of the record (that is to say, in a varying density record, the distance along the film between successive maximum or minimum densities) co-operating with a slit is equal to the width of the slit, the overall effect upon' the beam of light transmitted through the record is zero, that is to'say the response curve falls to zero. As the ripple wave-length is reduced, the response curve rises to a secondary maximum which, although considerably below the height of the response curve at low frequencies, is, nevertheless, of considerable importance. With further reduction in ripple wave length, the response curve falls again to zero and then rises to a tertiary maximum and so on, the amplitude of successive maxima gradually falling. The same effect exists when reproducing from a varying breadth record, the ripple wave-length in that case being the distance between successive peaks. It is an object of the present invention to reduce the magnitude of the secondary maximum. In any photographic record, there will be found to be a series of absent frequencies corre spending to the frequencies at which the response characteristic, in making the record, fell to zero. According to the present invention, the frequency at which the secondary maximum of be necessary to use a line of infinitesimal width,

, and

the reproducing characteristic occurs is made substantially to coincide with the lowest absent frequency of the record. Alternatively, the frequency at which the first zero of the reproducing characteristic occurs is made substantially coincident with the frequency of the secondary maximum of ithe record.

This may be accomplished by making the width of the slit used in reproducing from a photographic record diiferent from the width of the slit which was used in making the record. One of the slits is preferably l times as wide as the other. The ratio of the widths may vary somewhat from this figure but it should be between 1 and 1%. It should be understood that it is intended that'the word slit in this specification be given a wide interpretation as it is immaterial whether a physical slit in close proximity to the record is used or whether the effect of a slit is produced by optical means as, for example, by projecting upon the film an image of a slit placed at a distance from the film.

The invention will be better understood in the light of the ,following explanation: For simplicity, the description will be confined to records of varying intensity and constant breadth but it will be understood that the description is also applicable to records of varying breadth, the expression density of the record beingv replaced by breadth of the blackened portion of the record.

If a light beam, fully modulated at an acoustic frequency, falls through a slit of infinitesimal width upon a moving photographic film, the density of the record produced is represented by the equation: where m=a (cos wS+1) (1).

and therefore x cos wssin w ]+ad ..(2)

The modulation Z represented by equation (1) is unity and that represented by equation (2) is equal to 2a sin which may be written sin This is illustrated in the dotted curve A in the accompanying drawing, the response or modulation being plotted vertically and horizontally. Unit modulation is represented by the horizontal line a. The response is plotted in transmission units which are proportional to the logarithm of the modulation.

Since the same effect takes place upon reproduction, the overall degree of modulation, when the slit used is of the same width for reproducing as for recording, will be sin ' 2 rod This is shown in the plain line curve B of the drawing, in this case Z being plotted vertically in transmission units. As

aid

increases from o to 1r, Z falls gradually from unity to zero and as (Dd T is still further increased Z rises to a secondary maximum at a value of 1 2 slightly less than (where d1 is the slit width used in recording and d2 that for reproducing) equation (3) becomes and it will be found that the secondary maximum The overall response when using slits in recording and reproducing respectively in the ratio 3 :2 or 2:3 is shown in chain dotted curve C of the drawing, d being, in this case, the width of the wider slit. If curve C be compared with curve B, which is the equivalent curve for equal slits, it will be seen that in curveC the maxima and minima are less pronounced than in curve B.

I claim:

1. Apparatus for reproducing sound or other low frequency oscillations from a record of said oscillations with the aid of light comprising a moving photographic record, a light source, and a slit device, said device being so proportioned that the frequency at which the secondary maximum of the characteristic of the reproducing apparatus occurs is made substantially to coincide with the lowest absent frequency of the record.

2. Apparatus for reproducing sound or other 12 0 low frequency oscillations from a record of said oscillations with the aid of light comprising a moving photographic record, a light source, and a slit device, said device being so proportioned that the lowest frequency at which the response of the reproducing apparatus is zero is made substantially to coincide with the frequency of the secondary maximum of the record.

3. Apparatus for reproducing sound or other low .frequency oscillations from a record of said oscillations with the aid of light wherein a slit is used having a difierent width from that used in making the record, the width of one of said slits being in a ratio not less than 5:4 nor more than 7:4 to the width of the other of said slits.

4. Apparatus for reproducing sound or other low frequency oscillationsfrom a record of said oscillations with the aid of light wherein a slit is used having a different width from that used in making the record, the width'of one of said slits being substantially in the ratio 3:2 to the width of the other of said slits.

5. Apparatus for reproducing sound or other low frequency oscillations from a record of said oscillations with the aid of light wherein a slit is used having a difierent'width from that'used in making the record, the width of the slit used in making the record being in the ratio 3:2 to the width of the slit used for reproducing from said record. MICHAEL BOWMAN-LB. 

